412 REGINALD A. DALY 



tures above those at which even olivine can form. The "white" 

 heat of the huge lava-fountains corresponds to a temperature of 1300° 

 C. or over.' Granting that the Hawaiian conduits have always been 

 so much superheated, it is not surprising that so few types of lava, 

 other than olivine basalt, have been found on the island. 



Iddings has described a striking case of the sinking of the augite 

 phenocrysts, enriching a thick layer at the bottom of a 30-foot intru- 

 sive sheet on Electric Peak, Yellowstone Park.^ It is not clear why 

 so thin a sheet should have been differentiated and thus stand in 

 contrast to hundreds of mapped sills of at least as great thickness, in 

 which no differentiation is visible. Possibly the explanation is to be 

 found in the fact that the Electric Peak sill is unusually rich in sul- 

 phur trioxide, chlorine, lithia, and "combined" water — substances 

 which tend to lower the viscosity of magmas. In this case, though 

 the magma was rather quickly chilled against the inclosing shales, 

 the dissolved volatile matter maintained the fluidity long enough for 

 the phenocrysts to fall to the bottom. 



Against the hypothesis of fractional crystallization and gravitative 

 sinking, it might be objected that the heavy minerals of early genera- 

 tion in gabbros and other plutonic types which have crystallized 

 under slow-cooling conditions, are generally quite uniformly distri- 

 buted through rock and show no concentration by gravity. There 

 are, however, good reasons to beHeve that each plutonic magma, 

 before any crystallization begins, is regularly cooled down several 

 hundred degrees of the Centigrade scale below the melting-point of 

 the rock resulting from the crystallization of the magma. The 

 experiments of Oetling^ and Amagat^ show that pressure is a principal 

 cause of this retardation of crystallization. According to Velain^ 

 the retention of the volatile solvents, such as chlorides, would further 

 tend to lower the temperature of crystallization in depth, while their 

 escape at a volcanic vent would allow crystals to form at a higher 



^ Cf. H. Le Chatelier and O. Boudonard, High Temperature Measurements, 

 trans, by G. Burgess, New York, 1904, p. 246. 



2 Monograph 32, Pt. 2, U. S. Geol. Surv., 1899, p. 82. 



3 Tscher. min. u. petrog. Mitt., Vol. XVII, 1897, P- 332- 



4 Comptes Rendus, No. 16, 1893. 

 s Op. cit., p. 181. 



